Nowadays, low-frequency standard radio wave containing time data (that is, a time code) are transmitted in various countries (for example, Germany the United Kingdom, Switzerland, Japan, and so forth). In Japan, 40-kHz and 60-kHz low-frequency standard radio wave that have been subjected to amplitude modulation using a time code having a format shown in FIG. 11, are transmitted from two transmission facilities (located in Fukushima Prefecture and Saga Prefecture). The time code comprises a plurality of frame is defined to have a time cycle of 60 seconds. According to FIG. 11, the time code is transmitted in a frame every time the figure representing the minute of an accurate time is updated (that is every minute).
A radio wave clock that receives a time code. and corrects time data of a time circuit by the time code is known. In this kind of radio wave clock, there is comprised an AGC (Auto Gain Control) circuit that controls a gain of an amplification circuit, according to the intensity of the signal level after the amplification of the signal, output from the amplification circuit, so that the precise time can be corrected in an internal circuit even though the signal level of the received radio wave fluctuates.
In this AGC circuit, gain control of the amplification circuit was carried out by filtering the amplified signal. Therefore, a filter having a large enough time constant than the cycle of the modulation signal. Namely, because the cycle of the low-frequency standard radio wave is one second, a filter with a large time constant is necessary, and by this, a problem of a large delay until the transient operation of the AGC circuit becomes constant, occurs.
Furthermore, at a stage of actually constructing the whole circuit, the circuit needs to be designed, taking into consideration, several tens of seconds of delay, to prevent occurrence of ripples. By this, reduction in delay by contriving the filter included in the AGC circuit, namely, speeding up the AGC operation is difficult.
In a case where a weak radio wave is received by the radio wave reception device, it is difficult to carry out stable detection, due to noise, etc. included in the radio wave.
Furthermore, it is general that a filter for emitting noise is applied when carrying out detection for radio waves. Because a filter has a constant -pass band, the filter allows noise components that are close to the frequencies that are to be allowed to pass through, to also pass through. If the pass band is narrowed, time delay occurs. and effected the signal processing, etc., thereafter.